The invention relates to a method for limiting a boost pressure of an internal combustion engine which is supercharged by means of a turbocharger, wherein a desired boost pressure is compared with a limit value which is determined for the various engine and turbocharger operating conditions and a gas mass flow rate upstream of the compressor of the turbocharger on the basis of a simulation model.
In particular in turbocharged diesel engines, the following principle of boost pressure control is used if the charger is not a so-called self-regulating charger. By means of a boost pressure sensor, which is arranged at a suitable location in the intake section, the current actual value of the boost pressure is continuously supplied to a control device, in particular a vehicle control unit or engine control unit. The control unit then continuously compares this boost pressure with a set value of the boost pressure in order to influence the boost pressure by means of a suitable actuator element in accordance with the magnitude and profile of the deviation of the set value from the actual value. The actuator element may be electrically and/or pneumatically operated. The boost pressure may be influenced, in particular, by adjustment at a waste gate of a turbine of an exhaust gas turbocharger or by variation of the turbine geometry, in particular, a variation of the position of the turbine guide vanes, depending on the type of exhaust gas turbocharger.
A desired value or desired values of the charge pressure are required for the regulating process. The desired value or the desired values are usually stored directly in the control unit as a function of the respective operating point, as a characteristic diagram or as a characteristic curve. The operating point is defined in particular by the engine speed and the engine load. The desired value can, if appropriate, be corrected for ambient influences or environmental influences such as, for example, temperatures and ambient pressure. The set value can, if appropriate, also be corrected dynamically or have a dynamic behavior in order to improve the transient response when rapid changes in the operating point occur.
When the desired values for the boost pressure are determined, it is necessary, in particular in full-load operating mode, to take into account various peripheral conditions which partially contradict one another. These peripheral conditions include, in particular:    The higher the boost pressure the more air or more gas is fed to the internal combustion engine and it can correspondingly output more torque and power.    If excessively high desired values are set for the boost pressures when there is a low air throughput rate at a compressor of the exhaust gas turbocharger—that is to say at low engine speeds—what is referred to as “pumping” of the compressor occurs. It is important to avoid this “pumping” for reasons of comfort, efficiency and durability of the exhaust gas turbocharger.    If excessively high set values for the boost pressures are set when there is a high air throughput rate at the compressor—that is to say at high engine speeds—over-speeding of the exhaust gas turbocharger occurs. Because of the risk of destruction of the exhaust gas turbocharger and possible further ensuing damage to the internal combustion engine, this state must be avoided under all circumstances.    The setting of excessively high boost pressures is also accompanied by a rise in the charge air temperature which occurs downstream of the compressor in the direction of flow. The charge air temperature should be limited depending on the particular design and the material used for the charge air passages extending to a charge air cooler which is provided in the intake duct. For example, elastomer hoses are used for the charge air passages. However, charge air system manufacturers only warrant their compressors up to a specific gas temperature at the compressor output, because, inter alia, of the temperature-dependent strength profile of the lightweight metal alloys which are used for the compressor wheel. At an excessively high temperature the compressor blades may otherwise be unacceptably stretched under the influence of the centrifugal forces at a high compressor speed and come into contact with the compressor casing, which could cause the turbocharger to fail.
Because of the physical conditions at the compressor, not only the boost pressure itself but also the compressor pressure ratio p2/p1—where the pressure p2 represents the absolute pressure downstream of the compressor in the direction of flow and the pressure p1 represent the absolute pressure in the intake section upstream of the compressor in the direction of flow—and the volume stream through the compressor are decisive for the risk of “pumping” the compressor and the risk of over-speeding of the exhaust gas turbocharger. This results from a typical compressor operating characteristic diagram which is represented by way of example in FIG. 1. In FIG. 1, the volume flow dV1/dt is plotted on the abscissa, and the pressure ratio p2/p1 is plotted on the ordinate. The curve a characterizes what is referred to as the pumping limit and the curve b characterizes the exhaust gas turbocharger rotational speed limit. In the rest of the description, more details will be given on FIG. 1. When the internal combustion engine is operating at a high altitude or with a soiled air filter associated with the intake section, the compressor pressure ratio p2/p1 rises as the absolute boost pressure p2 remains constant since in both cases the pressure p1 upstream of the compressor decreases. As a result, the risk of exceeding one of the two limits, the pumping limit or the exhaust gas turbocharger rotational speed limit, increases.
If the maximum boost pressures and thus the maximum boost pressure desired values are configured or selected on the basis of normal altitude and a clean air filter in such a way that the pumping limit and the exhaust gas turbocharger rotational speed limit can reliably be prevented from being exceeded even under the most unfavorable conditions, for example maximum altitude which can possibly be reached and air filter at the soiling limit, under normal conditions this would lead to unnecessary cutting of the possibly achievable torque values and power values of the internal combustion engine.
Instead, the boost pressure desired values in a control unit are typically corrected as a function of a correction variable. This correction variable may be, in particular, an atmospheric pressure or preferably, if a corresponding pressure sensor is provided upstream of the compressor in the intake section, it may be the pressure p1 upstream of the compressor in the direction of flow. This makes it possible, under normal conditions, to permit higher boost pressures and thus generate also higher torque and higher power. The boost pressures are reduced for operation at a high altitude by means of a correction relating to the atmospheric pressure, or for operation at a high altitude and/or with soiled air filter by a correction relating to the pressure p1 which occurs upstream of the compressor in the direction of flow, and/or the atmospheric pressure in order to avoid “pumping” of the compressor and/or overspeeding of the exhaust gas turbocharger.
This is known from German laid-open patent application DE 100 54 843 A1. Here, the maximum acceptable boost pressure is determined as a function of a pressure upstream of a compressor p1 and temperatures upstream and downstream of the compressor in the intake tract, and as a function of the engine speed. In order to determine the maximum acceptable boost pressure, the relationship between the air charge and engine speed in the form of a characteristic curve is required. Such characteristic curves are usually determined in test bench trials on an internal combustion engine along the full-load under normal conditions and stored in a control device. If, for example, due to operation at a high altitude, the necessary desired value for the boost pressure is above the maximum acceptable boost pressure which is determined according to the disclosure of DE 100 54 843 A1 and stored in a control device, the control device will only set the maximum acceptable boost pressure within the scope of a minimum value selection. If, on the other hand, the maximum acceptable boost pressure limit is higher than the necessary desired value of the boost pressure, for example at normal altitude, this boost pressure is of course not limited by the maximum acceptable boost pressure limit.
The subject matter of German laid-open patent application DE 100 54 843 A1 ultimately attempts to depict the limiting curves for the pumping limit and the exhaust gas turbocharger rotational speed limit (see FIG. 1). The determination of the desired value for the boost pressure is based on a full-load characteristic curve for the air charge which is determined as an exemplary curve under normal conditions or on a full-load air charge profile which is determined by exemplary measurements under normal conditions and plotted over the rotational speed of the internal combustion engine and the corresponding pressures and temperatures or the corresponding state variables. Since the characteristic curve is determined by way of example on a test bench internal combustion engine or sample internal combustion engine under normal conditions along the full-load curve plotted against the rotational speed, values which are determined by means of the characteristic curve, for example a desired value for the boost pressure, may deviate from the values which are actually necessary since, in reality, the actual air charge value or air charge    varies from one internal combustion engine to another (variation between different examples of the same type of engine),    does not remain constant over the operating period as a result of soiling of the air-conducting parts (intake ducts, intake manifold etc.) and as a result of actuation or adjustment of the actuation times, for example as a result of lengthening of a valve drive operating chain or of a toothed belt, and also    is dependent not only on the rotational speed but also on further internal-combustion-engine-related variables such as, for example, a load, a cooling water temperature, an exhaust gas backpressure which is relevant in particular when operating with a soot filter, and leakage of an exhaust gas recirculation valve if an exhaust gas recirculation system is provided. Such leakage is relevant in particular for conventional operation without full-load exhaust gas recirculation.
This list is not exclusive. If the criteria listed above are not taken into account, they must be accounted for by selecting the desired values for the boost pressures in such a way that a correspondingly large safety margin is maintained between the limits to be observed (pumping limit and rotational speed limit of the exhaust gas turbocharger, see FIG. 1) at the expense of the optimum rotational speed values and power output values. In particular the possible influence of the exhaust gas backpressure which, during operation with a soot filter, fluctuates within a very large range as a function of the momentary charging of the soot filter, makes appropriate use of the method disclosed in DE 100 54 843 more difficult for applications with a soot filter.
The successful use of a method for limiting the boost pressure which uses the air charge value curve or air charge curve of the internal combustion engine which is determined under full-load at normal altitude is also questionable at an altitude above the normal altitude since at an altitude above the normal altitude the full-load curve of the internal combustion engine corresponds to a lower load than at normal altitude because the charge is load-dependent. This load dependence is however less and less pronounced as the rotational speed increases. In addition, the operating points of the exhaust gas turbocharger which are changed as a result of the change in altitude also provide values for the exhaust gas backpressure which are different from those obtained at normal altitude. This also has an influence on the air charge value curve.
Furthermore, the method for limiting the boost pressure according to German laid-open patent application DE 100 54 843 A1 is unsuitable for internal combustion engines with full-load exhaust gas recirculation for the following reasons.
In the customary embodiment of an exhaust gas recirculation system, in particular in the form of what is referred to as a “high-pressure exhaust gas recirculation system”, the exhaust gas is taken at a specific pressure upstream, in the direction of flow, of a turbine of the exhaust gas turbocharger located in the exhaust section, and is fed into the combustion air stream with a specific boost pressure downstream of a charge air cooler which is typically provided in the intake section. Opening the exhaust gas recirculation system when operating at full-load reduces both the stream of exhaust gas through the turbine and the stream of air through the compressor because part of the engine charge then is derived from the flow of exhaust gas which is recirculated and which is already branched off upstream of the turbine and returned to the intake air downstream of the compressor. In the disclosure in DE 100 54 843 A1 an exhaust gas recirculation system is neither provided nor taken into account.
Although an exhaust gas recirculation cooler can be provided in an exhaust gas recirculation line, the exhaust gas which is fed into the internal combustion engine from the exhaust gas duct via the recirculation system is typically hotter than the charge air downstream of the charge air cooler. As a result the fresh air is heated in the intake manifold or in the charge air distributor line. This constitutes a difference from operation without recirculation of exhaust gas. The heating of the charge air also causes the internal combustion engine to take in a smaller mass flow of gas. This in turn reduces the volume flow at the compressor.
DE 100 54 843 A1 does not disclose an exhaust gas recirculation line. If the method from DE 100 54 843 A1 were to be used in an internal combustion engine with exhaust gas recirculation, this would lead to a situation in which the compressor operating point would not be correctly detected at least with respect to the abscissa in the compressor characteristic diagram (FIG. 1). This could in turn lead to the limits for the “pumping” and/or the exhaust gas turbocharger limiting rotational speed being unintentionally exceeded.
Of course, air charge profiles which have been determined during operation with a specific exhaust gas recirculation rate which applies, for example, for normal conditions, could be stored in the control device. Any deviation from this exhaust gas recirculation rate, whether due to inaccuracies in the exhaust gas recirculation control or due to intentional reduction in the exhaust gas recirculation rate at an altitude higher than normal, would result in a change in the volume flow through the compressor which would not be detected or would not be taken into account by the characteristic diagram. This could result in the limits for “pumping” and/or the limiting rotational speed for the exhaust gas turbocharger being unintentionally exceeded. This would be avoided by selecting a boost pressure set value which ensures a correspondingly generous distance from these limits, however at the expense of reduced torque and thus reduced power.
An excessively large ratio between the boost pressure downstream of the compressor and pressure upstream of the compressor is also accompanied by a rise in the charge air temperature downstream of the compressor. The maximum permissible charge air temperature has to be limited depending on the design and material of the following charge air paths as far as the charge air cooler (for example elastomer hoses), for safety and durability reasons, but, if appropriate, also because of the charge air temperature limit for which the manufacturer of the supercharger has designed and warranted the compressor. The maximum value of this ratio, at which the maximum acceptable charge air temperature is exceeded, depends on several factors:    on the compressor volume flow rate which co-determines the compressor efficiency,    on the efficiency of the compressor (compressors of, for example, a different design and “quality” may also have different characteristic efficiency diagrams) because the lower this efficiency the higher the charge air temperature at constant values of the compressor volume flow and the ratio of the boost pressure to the pressure upstream of the compressor,    on the configuration of the charge air path from the compressor to the charge air cooler, in particular on the selection of material, and    on the temperature upstream of the compressor because the higher said temperature the lower the ratio of boost-pressure to pressure upstream of the compressor at which the acceptable air charge temperature is reached or exceeded will be.
In applications in which there is a trend for    the rotational speed limit of the exhaust gas turbocharger is rather high,    the compressor efficiency is not very high,    the, for example, material-based limitation of the charge air temperature are releasable lower and    the anticipated areas of use include areas with high temperatures, (for example Death Valley in the USA with air temperatures of up to 60° C.), and    the maximum desired boost pressures are comparatively high (that is to say especially in the case of what are referred to as “supercharged systems”),it may be found that, at least starting from a certain air temperature upstream of the compressor in the direction of flow, the maximum acceptable charge air temperature is reached earlier than the rotational speed limit for the exhaust gas turbocharger. Typically, the maximum acceptable boost pressures are limited more severely as a reserve or as a safety measure for situations where the ambient air or intake air than would be necessary because of the rotational speed limit of the exhaust gas turbocharger in order to protect the materials, for example elastomer hoses between the compressor and the charge air cooler or also to protect the compressor itself.
When the internal combustion engine is used at normal temperatures, this limitation would result in a boost pressure, torque and thus power of the internal combustion engine being limited more severely than would be necessary for this operating situation. As an alternative to limiting the boost pressure it is typically also possible to limit the ratio of boost pressure to pressure upstream of the compressor.
German laid-open patent application DE 101 22 293 A1 also discloses a method for controlling a boost pressure limitation of a turbocharger in an internal combustion engine in which the actual boost pressure is respectively determined after specific time internals and is compared with predefined values for the desired boost pressure in the respective operating state from a stored compressor characteristic performance graph, and adjusted.
There is however a disadvantage in that the temperature occurring upstream of the compressor is not taken into account when the maximum acceptable pressure ratio of the supercharger is determined—and thus also when the maximum acceptable boost pressure is determined. It is thus possible for the temperature occurring downstream of the compressor to exceed a maximum acceptable value, and for components such as, for example, elastomer hoses or the compressor itself, to be damaged under hot ambient conditions even before the maximum acceptable boost pressure for the rotational speed limit of the exhaust gas turbocharger is reached.
It is the object of the present invention to provide a method which is improved over the prior art methods, such that the boost pressure of an internal combustion engine which is supercharged by means of an exhaust gas turbocharger is limited under any condition only to the extent as required by those conditions.